It is often necessary or desirable for a person to exercise a particular muscle or group of muscles. For example, when a muscle is damaged, such as through injury or surgery, it is important to exercise the muscle to prevent atrophy and to strengthen the muscle for normal use. Further, people exercise healthy muscles to increase strength and to maintain an active and healthy lifestyle, as well as to improve their appearance. Various routines have been developed to exercise different muscle groups by forcing the muscles to contract and extend under a load, such as by moving a free weight against the force of gravity or by moving a handle whose movement is resisted by an exercise machine.
One such exercise is known as a shoulder press. An exerciser sits upright on a seat and grasps a barbell at shoulder level. The exerciser then pushes the barbell upward, extending his arms, and lowers it down. This exercise can be dangerous as the exerciser may drop the barbell. Further, the exerciser should have a partner to spot him in case he fails to lift the weight. When using free weights, the resistance provided by gravity is constant while the strength of the muscles varies over the range of motion. Consequently, the muscles are not fully loaded at each point over the range.
During a shoulder press, the hands seek to follow a curved path inward as the weight is extended upward. This path cannot be followed when using a barbell because the hands are maintained at a fixed distance. Further, the user's head must be tilted back or forward during the lower portion of the stroke to clear the barbell. These deficiencies can be overcome by performing the exercise with dumbbells.
To overcome these difficulties, machines have been developed that simulate the exercise movements of a shoulder press. In one apparatus marketed by the assignee of the instant application, a user exercises by pushing handles upward from shoulder height while in a sitting position. A seat and backrest are mounted to a frame to position a user. Two arms are connected together and rotatably mounted to the frame. The handles are mounted to the arms. The pivot for the arms is disposed behind the seat at about shoulder level. A cable operably connects the arms to a weight stack such that when a user pushes on the handles, thereby rotating the arms, the weight stack is lifted and provides resistance to the exercise. The cable may extend over a variable radius cam which alters the distance the weight is displaced for a given amount of handle rotation. In this configuration, the resistance to the movement of the handles can be varied to match the strength curve of the back and shoulder muscles. While this apparatus has solved many problems associated with performing a shoulder press exercise with barbells or dumbbells, it does not permit the user to vary the distance between his hands while performing the exercise.
In another apparatus, disclosed in U.S. Pat. No. 5,044,632, an exercise machine is disclosed in which levers are rotatably mounted to a frame above the seated user. Handles are mounted to the levers. Resistance to handle movement is provided by weight plates mounted to the levers. The hinges for the levers are disposed at an angle of 5.degree. with respect to a central vertical midplane, such that the user must move his hands in defined arcs in diverging planes as he presses upward on the handles. This apparatus forces the user's hands to follow a preset rate of convergence and divergence during the exercise stroke, regardless of the user's anatomy. This apparatus does not permit the user to select his own path of motion for the press exercise. Rather, the motion is dictated by the angle of the hinges.
A shoulder exercise apparatus is disclosed in U.S. Pat. No. 4,603,856. In this device, a bench is provided for the user to exercise in a prone or supine position. A shaft extends from a ball and socket joint mounted to the side of the bench, and a handle is slidably mounted to the shaft. Frictional resistance is provided both at the ball and socket joint and at the sliding connection between the handle and the shaft. The user exercises by moving the handle against one or both of these resistances. While providing multiple paths of motion through the range of the ball and socket joint, this machine provides for exercising only one arm at a time, cannot coordinate the motion of two arms, and has the disadvantages associated with frictional resistance such as changing resistance due to heat buildup, and wear. Further, this machine only provides concentric action (i.e., where the muscles contract against a load). No eccentric action (i.e., where muscles extend under a load) is possible with this machine.